Some industrial vehicles, such as track-type loaders, having hydrostatic transmissions utilize the transmission for dynamic braking and employ a separate friction brake to hold the machine once the vehicle has been stopped. Such vehicles frequently have a delay valve device in the control system to ensure that the friction brake is not applied until the vehicle has come to a complete stop. Such control systems also frequently have a shunt valve therein to short circuit the closed loop between the pump and motor thereof as a further measure to ensure that the hydrostatic transmission is completely neutralized anytime the friction brake is applied. Such a system is disclosed in U.S. Pat. No. 3,341,073 issued to William J. Spivey, Jr. on July 27, 1982 and U.S. Pat. No. 4,343,150 issued to Shuler et al on Aug. 10, 1982.
While such controls function satisfactorily in most work situations normally associated with track-type loaders, there is at least one work situation encountered in which the delay in applying the friction brakes causes some minor inconveniences. For example, if the vehicle is being stopped on a slope, the delay in applying the friction brakes after the transmission has been neutralized can allow the vehicle to roll down the slope from 2-9 centimeters before the friction brakes engage due to internal leakage in the pump and motor of the hydrostatic transmission. Similarly, while starting vehicle motion from a brakes applied stopped position, the process of releasing the friction brake and simultaneously engaging the hydrostatic drive can permit the vehicle to roll down the slope from 2-30 centimeters before the vehicle can respond completely to operator inputs. While such roll is not normally noticeable or objectionable, it can be an inconvenience if the operator is trying to stop the vehicle at a very precise location on the slope or start the vehicle moving without it rolling down the slope before the transmission is actuated.
The present invention is directed to overcoming one or more of the problems as set forth above.